One method far recording/reproducing information data as a pit train on/from an optical disk, is one in which data having a value of [1], for example, are recorded corresponding to the leading edge or trailing edge of a pit and in which the original data are reproduced by detecting these edge positions within a reproduced signal waveform. This mark edge recording method can be used to make a higher density recording than a "pit position" method in which the pits are recorded to correspond to the data [1] irrespective of pit edges.
The data recording/reproducing method by the mark edge recording method will be described with reference to FIG. 15. Here, the description will be made corresponding to the case in which the user data are modulated by the encoding method having the minimum run length such as a (2, 7) RLL modulation at 2. If the decode window is indicated by T, the smallest pit length and space length are at 3T. Moreover, it is assumed that the reproduced channel characteristics to be determined by the optical disk and the reading optical spot are PR (1, 2, 3, 3, 2, 1). This implies that when a data string "---0001000---" are recorded in the optical disk, the reproduced response waveform takes a format of "---01233210---". When the encoding method having the minimum run length at 2 is adopted, the level of the reproduced signal to be taken at a data recognition point is understood to take eleven levels of 0, 1, 2, 3, 4, 6, 8, 9, 10, 11 and 12 by superposing the PR (1, 2, 3, 3, 2, 1) characteristic. The levels 5 and 7 do not appear due to the run-length limitation.
If similar operations are performed for a random pattern, there is obtained an eye pattern, as shown in FIG. 14(a). The PR (1, 2, 3, 3, 2, 1) characteristic has a wide range of response waveform so that the eye takes a small opening height. This eye pattern is prominently different from eye patterns of the PR (1, 3, 3, 1) characteristic, the PR (1, 2, 2, 1) characteristic and the PR (1, 2, 1) characteristic, as shown at (b), (c) and (d) in FIG. 14. In any of these cases, however, the edge positions, as obtained by slicing at the center level, are concentrated at one point so that the data can be reproduced by detecting the edge position.
By inverting the polarity of recording data 2001 with the data [1], a recording data pulse train 2002 is obtained. By subjecting this pulse train 2002 to a (not-shown) suitable recording waveform control, a recording mark train 2003 is recorded on the disk. By scanning the recording mark with a reading optical spot, a reproduced signal 2004 is obtained. This reproduced signal 2004 is made binary as pulses 2006 along a suitable slice level 2005. At the timings of the rising and falling edges of the binary pulses 2006, a pulse signal 2007 is obtained. This pulse signal 2007 is fed to a PLL circuit (i.e., a phase locked loop circuit, not shown) so that a synchronized clock signal 2008 is outputted from the PLL circuit. As shown, the rising edge of the pulse signal 2007 and the rising edge of the lock signal 2008 are synchronized. By latching the pulse signal 2007 with the falling edge of the clock signal 2008, read data pulses 2009 are obtained. If the read data are made to correspond to the data [1] when the read data pulses 2009 are at "H", there are obtained reproduced data which are identical to the recording data.
There has also been proposed another method. The pulse signal 2007 is divided into rising/falling pulse trains, and these pulse trains are operated independently of each other. After this, the leading edge data and the trailing edge data are recomposed by absorbing their timing discrepancy with a synchronous pattern. Japanese Patent Laid-Open No. 212718/1996 is enumerated as prior art of this kind.
A conventional method other than the data recording/reproducing method shown in FIG. 15 will be described with reference to FIGS. 16 and 17. The operations corresponding to obtaining a recording data pulse train 2102 from recording data 2101 to that of obtaining a reproduced signal 2104 from a recording mark train 2103 are identical to those of FIG. 15. According to this method, the reproduced signal is sampled with a clock signal, and the edge data are extracted, while synchronizing the clock signal with the reproduced signal 2104, from the data of a sampled data train 2105. These operations will be described with reference to FIG. 17. A reproduced signal 2201 is sampled with a clock signal 2208 in an A/D converter 2202. The sampled data are cleared of the inter-symbol interference by a digital equalizer 2203 and are equalized according to the PR (1, 2, 3, 3, 2, 1) characteristic. After this, the sampled data are cleared of the amplitude fluctuation and the level fluctuation by an amplitude/level compensator 2204 and are fed to a data decider 2205. These operations are wholly carried out in synchronism with the clock signal 2208. The sampled data 2209, having passed through the amplitude/level compensator 2204, are fed to a phase comparator 2206, in which the sample timing discrepancy of the A/D conversion is detected in terms or a voltage, so that the clock signal 2208 is obtained by controlling the oscillation frequency of a VCO (Voltage Control Oscillator) 2207. In the data decider 2205, whether or not the sampled data are the edge data is decided with two threshold levels 2106 and 2107, and reproduced data 2210 are outputted. IN other words, reproduced data 2108 identical to the recording data 2101 are obtained by deciding the sampled data between those levels at [1] and the remaining data at [0].
The method of FIG. 15 and the method of FIG. 16 will be discriminated by calling the former the analog operations and the latter the digital operations.
The prior art, as described with reference to FIGS. 15 and 16, are sensitive to the level fluctuation and the amplitude fluctuation of the reproduced signal. As means for improving the reliability, therefore, there has been devised a technique of compensating the slice level. The analog operations are disclosed in Japanese Patent Laid-Open No. 81324/1990 or No. 254514/1987, and the digital operations are disclosed in Japanese Patent Laid-Open No. 263943/1996.